Cooling argon particle model

Marking points

1. 1 Describe that, as argon cools, its particles slow down because their kinetic energy decreases.
2. 2 Describe the gas-to-liquid arrangement change: particles go from spread out to close together or touching.
3. 3 Describe the gas-to-liquid movement change: in a liquid, particles still move but more slowly and change direction more often than in a gas.
4. 4 Describe the liquid-to-solid arrangement change: particles become fixed in a regular pattern.
5. 5 Describe the liquid-to-solid movement change: particles no longer move freely and vibrate about fixed positions.
6. 6 Link at least one arrangement or movement change to forces or energy changes, such as attractive forces becoming important as particles get closer or stronger in the solid.

Full-mark answer

As the argon cooled, the kinetic energy of its particles decreased so the particles moved more slowly. In the gas the particles were spread far apart and moved freely. When the argon became a liquid, the particles were closer together or touching, still randomly arranged and still moving, but more slowly and changing direction more often. When it became a solid, the particles formed a regular arrangement in fixed positions and only vibrated about those positions. The particles became closer together as attractive forces between them became more important, and these forces are stronger in the solid than in the liquid.